It is well known in the art that lean fuel/air mixtures may be used advantageously to produce relatively low levels of exhaust emissions and relatively low fuel consumption. However, drivability often suffers when lean mixtures are employed, because mixtures which are slightly "too lean" result in a markedly increased incidence of combustion instability. Combustion instability results in poor drivability and increased emission of hydrocarbons.
A measurement of engine acceleration has been shown to be a reliable source of information regarding combustion instability. Engine acceleration may be measured by monitoring changes in the speed of the engine, or by measuring the motion of the engine. Further, it has been shown that control systems may be constructed which continuously urge the fuel/air mixture leaner, until an indication of "over-leanness" (combustion instability) is detected at which time the mixture is urged richer. However, these systems require the mixture to be over-lean for a period of time before the mixture responds to the enrichening.
Control systems for automobile engines must be especially fast and accurate to provide the correct quantities of fuel and air at each moment in time, while the engine experiences rapid changes in throttle position, speed, and load. The ability of a control system to quickly adjust its mixture based on changes in operating conditions is characterized as overall system response. Therefore, a need exists to minimize the response time for a lean burn mixture control system.
Fuel injection has been used commercially for many years. The term "fuel injection" is used whenever the liquid fuel is forced into the engine under pressure, as opposed to aspirated into the airstream, as done by a carburetor. Fuel injection may be either mechanically or electronically controlled, and the flow of fuel may be either continuous or intermittent.
In all of these forms, it is desirable to control the air-to-fuel ratio. When all the cylinders of a multi-cylinder engine are treated as an aggregate; the control systems are designed to deliver the same air-to-fuel ratio to each of the individual cylinders. In fact, one of the chief benefits claimed for many prior art fuel injection systems is their ability to deliver nearly the identical quantity of fuel to all the cylinders.
While uniformity of fuel delivery to all cylinders is an object of most prior art systems, air delivery to each of the cylinders is not necessarily exactly uniform, and the air-to-fuel ratio requirements of each cylinder are not necessarily uniform, particularly when lean mixtures are used.
The prior art relies on a plurality of sensors to measure engine and environmental operating conditions. The control systems described in the prior art make control decisions based on sensor inputs. Each of those prior art sensors measures a time-averaged signal which is assumed to represent all the cylinders as an aggregate. Information regarding the differences between cylinders is not available from those prior art sensors. Accordingly, there is a need for control systems which consider separately the conditions in each cylinder.
Exhaust Gas Recirculation (EGR) has been used for many years for exhaust emission control purposes. One of the disadvantages of prior art EGR systems is degradation of drivability. It is well known that EGR has a tendency to spoil the combustion. EGR is known to reduce combustion temperatures, reduce the speed of flame propagation within the combustion chamber, and increase cyclic variability from one combustion event to the next. This variability of flame speed causes undesirable combustion roughness, which is perceived by the driver of the vehicle as poor drivability.
When EGR is used with prior art lean burn control systems, the degradation in drivability can be particularly offensive. Therefore, there exists the need for a lean burn control system which is compatible with EGR.
Water injection has also been known (and sometimes used) for many years for purposes of engine knock suppression and power enhancement. Prior art water injection systems have sometimes substituted alcohol or a water/alcohol mixture for water, to prevent freezing in cold weather. When water or alcohol are injected into the intake manifold in quantities roughly equal to the fuel flow rate, the principal effect is a cooling of the combustion gasses. As a result, the formation of Oxides of Nitrogen (NO.sub.x) in the combustion gasses is reduced markedly, and the intake charge is cooled. Water injection has been used to increase the density of the intake charge, allowing more power to be achieved without producing excessive engine temperatures. In smaller quantities, water injection is an effective emission control technique for NO.sub.x reduction.
However, as with EGR, water injection tends to spoil the combustion and reduce the engine's tolerance for lean mixtures. Water injection is known to reduce combustion temperatures, reduce the speed of flame propagation within the combustion chamber, and increase cyclic variability from one combustion event to the next. This variability of flame speed causes undesirable combustion roughness, which is perceived by the driver of the vehicle as poor drivability. Therefore, a need exists for a lean burn control system which is compatible with water injection.